Wastewater liquids flowing into treatment plants consist of approximately 98% by volume soluble and 2% non-soluble mixture. The 2% non-soluble portion causes the major problems found in liquid pumping applications for wastewater treatment plants. Common pumps used in these treatment plants are centrifugal and positive displacement type pumps.
A centrifugal pump operates on the principle of adding energy to the liquid by an impeller revolving at between 750 and 3000 revolutions per minute. Wear and premature failure of the volute and impeller is created by grit impacting those components at high velocity. Stringy materials in the wastewater regularly become wrapped around centrifugal pump impellers which can stop the pump or greatly reduce pump flow. This type of pump is also limited to flooded suction conditions and must be protected from running dry such as when emptying a tank. Mechanical seals or packing is required to prevent leakage of the pumped liquid from exiting through the rotating shaft and casing. Another disadvantage of centrifugal pumps is that because flow is not proportional to pump speed an external flow meter is required to vary flow rates.
Current diaphragm pump designs utilize a single diaphragm that is deflected by means of a piston or rod attached to the center. The problem with this design is the diaphragm must be able to withstand continuous differential forces acting on the diaphragm material. When the diaphragm moves to the up stroke position, the forces acting on the underneath side of the diaphragm is low and most likely a vacuum or negative pressure is created. The diaphragm material must resist imploding and is in a compressive state.
Once the stroke is reversed and begins moving down, the diaphragm must overcome the discharge pressure. The forces acting on the bottom of the diaphragm are positive and the diaphragm material must resist expansion and is in a state of tension. The greater the discharge pressures the greater the differential forces on the diaphragm material. For example, if the pump is operating at 300 strokes per minute at a discharge pressure of 25 psig and is under a suction lift of 2 psig, then the diaphragm material will see a pressure swing of 27 psig every one fifth of a second. This causes fatigue on the material which leads to failure due to tearing of the material.
The larger the diaphragm and the higher the discharge pressure the shorter the life expectancy of the diaphragm. For this reason the size of the diaphragm for rod driven diaphragm pumps is kept small in size and less than a 1xe2x80x3 stroke length. Increasing the thickness of the diaphragm to increase discharge pressure will also increase the diaphragm""s rigidity causing the same failure. Decreasing the thickness adds flexibility but decreases the pump performance for discharge pressure. The present invention is based upon the operating principle of gas, which being compressible, acts according to the formula P1V1=P2V2.
There are two types of positive displacement pumps. The first is a close tolerance pump that relies upon close fitting parts to displace a volume fluid by means of a piston, gear, or progressive cavity. These pumps are highly susceptible to wear caused by grit. As the tolerances diminish between the moving parts, flows will also decrease and the pump speed must be increased to compensate for the loss. This in turn accelerates the deterioration of the pump until the flow is below required performance for the application. Rags are also concern because pump failures occur from them becoming lodged in between the rotor and stator. This pump is also limited to flooded suction conditions and must be protected from running dry such as when emptying a tank. Mechanical seals or packing is required to prevent leakage of the pumped liquid from exiting through the rotating shaft and casing. The footprint of the pump is large in relation to the performance requiring a larger area for installation than other types of pumps.
The other type of positive displacement pump is a diaphragm pump. Its principle of operation is to displace volume by a diaphragm in a reciprocating motion. In order for liquid to move in one direction by the displaced volume, check valves are required. Check valves are located on the inlet and discharge side of the pump. This type of pump has limited flow rates and discharge pressures due to the design of the diaphragm. The use of a single diaphragm greatly limits the size and displacement stroke due to the need of flexibility for movement and rigidity for creating the discharge pressure. The reciprocating motion also imposes differential pressures on the diaphragm material ranging from a negative pressure on the up stroke to a reversing situation on the down stroke, which is a positive pressure. This is a limiting factor due to the cause of diaphragm failure and thus limits the applications for its use. Also, the check valves are an essential component for the workings of the pump. If a check valve fails to seat properly, then all flow is stopped. Stringy material and grit are common causes of this problem and are high maintenance for treatment plant operators.
A positive displacement pump utilizing diaphragms and check valves can be utilized in many applications beyond simply treatment plants, however, treatment plants have particularly aggressive environments that can cause rapid failure of equipment. Development and production of pumps capable of extended life at treatment plants will undoubtedly create demand for similar types of long lived, scalable pumps in other industrial settings.
For the foregoing reasons, there is a need for a scalable pump capable of moving liquids with non-soluble constituent that is not subject to failure based upon the abrasive effects of the non-soluble components or the damaging effects of stringy and cloth type materials.
The present invention is directed to a positive displacement pump that satisfies this need of providing a pump capable of withstanding the damaging effects of liquids containing grit and fiber that cause rapid wear in centrifugal and close tolerance positive displacement pumps. In addition, the present invention is directed to a positive displacement pump that satisfies the need of minimizing the deleterious effects of rapid pressure reversals on the diaphragms that are utilized in these pumps.
A pump apparatus having features of the present invention comprises an internally pressurized diaphragm assembly positioned atop and secured to a pump bowl. When the pump bowl is flooded with a liquid the internally pressurized diaphragm assembly is capable of applying a negative pressure to the liquid at the suction inlet and a positive pressure to the liquid at the discharge outlet. Check valves positioned within the suction inlet and discharge outlet restrict movement of the liquid to a single direction such that during a diaphragm up-stroke, when negative pressure is applied, the liquid is drawn into the pump bowl from the suction inlet, however, liquid cannot be drawn back in from the discharge outlet because check valve restricts the flow. When the internally pressurized diaphragm undergoes a down-stroke and the assembly produces a positive pressure, the liquid is forced into the discharge outlet. This liquid, however, cannot escape back through the suction inlet under positive pressure because the check valve restricts flow to a single direction.
The internally pressurized diaphragm assembly is comprised of an upper and lower diaphragm preferably comprised of nitrile utilizing a reinforced vulcanized nylon mesh or similarly elastic yet durable material, an upper diaphragm plate positioned atop the upper diaphragm along with a lower diaphragm plate positioned beneath the lower diaphragm. The diaphragms are secured, in an air-tight fashion, to the pump bowl proximate to their outer periphery with the aid of an outer diaphragm ring and a spacer ring. The upper and lower diaphragm plates have a smaller diameter than the upper and lower diaphragms and when secured in position leave exposed an annular portion of the upper and lower diaphragm. The annular portion is further defined by the laterally outward projection of the annular portion of the upper and lower diaphragms resulting in an internal cavity.
The cavity of the diaphragm can be pressurized by means of a valve or other mechanism to a predetermined level. The pressurization of the annular cavity diminishes the damaging effect of the differential forces acting on the diaphragm assembly. Positive displacement pumps utilizing a single unpressurized diaphragm are especially susceptible to premature failure as the diaphragm is subject to negative pressure (compression) when in the up-stroke position and positive pressure (tension) when in the down-stroke position. Rapid fluctuations in these countervailing pressures during normal operation of a single diaphragm positive displacement pump and the resulting diaphragm fatigue are the principal cause of pump failure. The implementation of a pressurized diaphragm assembly eliminates the swing from compression to tension of the upper and lower diaphragms thereby increasing the life expectancy of the pump assembly.
Accordingly, it is the primary object of the present invention to provide a pump that can easily pass solids without clogging, wearing, or causing failure to the moving parts exposed to the liquid being pumped. Another object of the present invention is to enable operation without damage during run dry conditions, such as draining a tank. A still further object of the present invention is to eliminate the need for mechanical seals or packing. A still further object of the present invention is to produce flow rates proportional to pump speed in order to maintain a desired flow rate that is linearly adjustable by changing rotating speed up to the design requirements of the application. A still further object of the present invention is to maintain a desired flow rate without variations due to discharge pressures up to the design requirements of the application. A still further object of the present invention is for all wearing parts to be easily accessible for maintenance. A still further object of the present invention is operation with low shear conditions for liquid solution passing through pump.
Additional objects and advantages of the invention are set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claim.